1. Field of the Invention
The present invention relates to electrophoretic migration imaging apparatus and procedures and more particularly to improved apparatus and method for image exposure in electrophoretic migration imaging systems.
2. Description of the Prior Art
Electrophoretic migration imaging processes capable of producing polychromatic images have been extensively described in the patent literature. Early publication of these processes occurred in a series of patents by E. K. Kaprelian including U.S. Pat. No. 2,940,847 issued June 14, 1960; U.S. Pat. No. 3,100,426 issued Aug. 13, 1963; U.S. Pat. No. 3,140,175 issued July 7, 1964; and U.S. Pat. No. 3,143,508 issued Aug. 4, 1964. More recent publications relating to polychromatic, electrophoretic migration imaging processes include U.S. Pat. No. 3,383,393 to Yeh issued May 21, 1968; U.S. Pat. No. 3,384,565 to Tulagin and Carriera issued May 21, 1968 and U.S. Pat. No. 3,384,566 to Clark issued May 21, 1968.
In a typical embodiment of a single-pass, polychromatic, electrophoretic migration imaging system, images are formed by providing a suspension of electrically photosensitive particles of three different color types (each type of particle being sensitive uniquely to a particular color of light) between a transparent, electrically conductive electrode (commonly termed the "injecting electrode") and an electrode bearing an electrically insulating layer on its outer surface (commonly termed the "blocking electrode"). An electric field is applied across the two electrodes while simultaneously exposing the particles to a multicolor light image which is selectively absorbed by the particles according to light color.
As these steps are completed, selective particle migration takes place in image configuration producing complimentary images on both electrodes. While the theory of image formation is not completely understood, it is believed that the particles initially bear a charge in the imaging suspension which causes them to be attracted to the injecting electrode upon application of the electric field between the blocking and injecting electrodes. Upon exposure to activating electromagnetic radiation to which they are sensitive (i.e., of a color absorbed), the exposed particles adjacent the injecting electrode apparently undergo a change in charge polarity by exchanging charge with the injecting electrode. These particles, now bearing the same charge polarity as the injecting electrode, are repelled by it and migrate to the blocking electrode. The particles which migrate to the blocking electrode are less able to exchange charge with that electrode's insulating layer and do not therefore readily recycle to the injecting electrode. As a result, an image is formed by particle subtraction on the injecting electrode, such image being typically a photographically positive image, and a complimentary image, typically a negative or reverse image, is formed on the blocking electrode.
Such a system offers substantial advantage by allowing exposure and development of all substituents of a color image at a single exposure zone, thus eliminating the need for three color separation exposures and three separate development stations. Perhaps even more attractive is the possibility of avoiding the problems associated with registering a plurality of color separation images.
Although the potential advantages mentioned above are significant, there are also substantial problems connected with the system. Two of the most serious are (1) the difficulty in obtaining high density images via exposure at a single exposure zone and (2) the difficulty in maintaining a proper proportional deposition of the plurality of different color pigments activated in a single exposure so as to obtain an accurately color balanced reproduction of the original.
Consideration of various prior art implementations of the electrophoretic migration imaging process emphasize these problems. For example, as a means to increase "image and color quality", U.S. Pat. No. 3,719,484 discloses a special nip exposure construction which subjects the photoelectrophoretic ink to a plurality of web electrodes within a single exposure zone. U.S. Pat. Nos. 3,703,335 and 3,667,842 are representative of apparatus in which a plurality of aligned, reinforcing-imagewise-exposures are projected at separate scan zones.
U.S. Pat. Nos. 3,857,549 and 3,682,628 and British Pat. No. 1,247,465 are examples of apparatus approaches which have forgone the advantages offered by a single exposure of a tri-particle dispersion and utilize three color separation exposures of three different single color suspensions. These approaches thus choose to deal with severe registration problems rather than the density and color balance problems mentioned above. It should be noted that U.S. Pat. No. 3,857,549 avoids registration problems; however this is accomplished only by using the original to be produced in contact printing relation with an electrode of the copy apparatus. This latter approach provides certain distinct machine limitations, e.g., with respect to handling of the original and to magnification changes. All such prior art apparatus suffer from the complexity and expense of three separate imaging and development stations.
U.S. Pat. Nos. 3,649,515 and 3,663,396 disclose apparatus for controlling color balance without registration of color separation images, U.S. Pat. No. 3,663,396 by using a color television tube as the exposure source and varying the intensity of its output, and U.S. Pat. No. 3,649,515 by forming color-correcting masks with a pre-imaging and development sequence. Commonly assigned U.S. application Ser. No. 740,699, filed Nov. 11, 1976 now U.S. Pat. No. 4,058,828 discloses correction for color balance by means of selective control of the color content and intensity of three co-linear color laser beams scanned across the imaging nip. The above techniques are helpful but additional improvement in the image density, color balance and other characteristics of such systems would be desirable.